In conventional electronic weighing apparatus, an AD (analog-to-digital) converter for converting an analog weight signal, outputted from a load cell (a weight detector), into a digital signal has hitherto been employed in the form of either a double integral model (See lines 13 to 14 in bottom left column on page 4 of the Japanese Laid-open Patent Publication No. 62-69125) or a successive comparison model. The former has a problem in that, when the resolution is desired to be increased, the response tends to be considerably lowered, whereas the latter has a problem in that the resolution is low although the response is high.
In view of the above, it has been suggested to use a delta.multidot.sigma-type AD converter (hereinafter referred to as a ".DELTA..SIGMA.-type AD converter") capable of giving high resolution (that is, a high accuracy) and having a response superior to that of the double integral model. This .DELTA..SIGMA.-type AD converter has a low-order modulator for effecting a voltage-frequency modulation of an analog signal and a digital filter both built therein and is inexpensive. The modulator used therein can output a small quantity of information (1 bit) at an extremely high rate while the digital filter used therein can accomplish a high resolution filtering at a low rate.
However, the digital filter built in the .DELTA..SIGMA.-type AD converter has a problem in that, since such a digital filter accumulates sampling data corresponding to the number of times over which the digital signal due to its peculiar sampling calculation has been inputted, a delay in time tends to occur in the output. Consequently, the response cannot be regarded sufficiently high. In particular, when the cut-off frequency of the digital filter referred to above is chosen to be low in order to increase the weighing accuracy accomplished by the .DELTA..SIGMA.-type AD converter, the amount of sampling calculation tends to increase, accompanied by an increased delay in outputting. This delay is expressed in terms of settling time. The lower the out-off frequency of the digital filter, the longer the settling time, and the longer settling time results in deterioration of the follow-up characteristic relative to an abrupt change of the input. Accordingly, if using only the digital filter built in the .DELTA..SIGMA.-type AD converter, the filtering is carried out to a level at which no practical problem occur, the cut-off frequency tends to be considerably lowered but the response tends to be correspondingly reduced.
In this type of electronic weighing apparatus, an amplifier is also employed to amplify the analog weight signal outputted from the load cell. This amplifier tends to exhibit a drift in its output signal with change in, for example, power source voltage and/or temperature. Once this drift occurs, a weighing error results and, therefore, the drift needs to be dealt with as one of the problems to be solved for the purpose of increasing the weighing accuracy.
One method of eliminating the drift is suggested in which a circuit path through which the analog weight signal outputted from the load cell is inputted to the amplifier is disconnected, and then the input terminals of the amplifiers are shortcircuitted with each other. This allows an analog-to-digital conversion of an offset voltage thereof to be effected thereby enabling the drift to be detected in reference to a change in time. However, the shortcircuitting of the input terminals of the amplifier tends to result in an abrupt change in input level of the .DELTA..SIGMA.-type AD converter as compared with the usual case in which the analog weight signal is inputted to the amplifier. If at this time the cut-off frequency of the digital filter employed in the .DELTA..SIGMA.-type AD converter for the purpose of increasing the weighing accuracy is lowered, a relatively long time is required before the output follows up. Accordingly, the length of time, during which an accurate output value indicative of the drift cannot be obtained tends to be prolonged significantly. Also, when the analog weight signal is inputted the second time, a comparably long time is required for the digital filter to restore to the level of the input. Consequently, the response of the digital filter is considerably lowered at the time of drift detection.
The present invention has been devised with due regard to the existing situations discussed above and has for its primary object to provide a weighing apparatus wherein an analog filter is incorporated in the .DELTA..SIGMA.-type AD converter to make it possible to set the cut-off frequency of the digital filter at a high value. Thus, the weighing apparatus can exhibit not only a favorable response characteristic, that is, a high weighing speed, but also an increase in weighing accuracy even though the cut-off frequency of the analog filter is set at a low value.
Another object of the present invention is to provide the weighing apparatus of a drift-compensated type wherein not only is a rectification of an error in the weight signal carried out in correspondence with the amount of drift of the amplifier thereby to increase the weighing accuracy, but the detection of the drift is quickly accomplished thereby minimizing the lowering of the weighing speed which would otherwise result from the drift correction.
Other objects and features of the present invention will become clear from the following description of the preferred embodiments of the present invention.